Thermal recording material

ABSTRACT

Disclosed is a heat-sensitive recording material comprising
         (a) a transparent film,   (b) a heat-sensitive recording layer formed on one side of the transparent film, and containing an electron-donating compound, an electron-accepting compound, and a binder,   (c) a protective layer formed on the heat-sensitive recording layer, and containing an aqueous resin as a primary component; and   (d) a backside layer formed on the other side of the transparent film, and containing a pigment and a binder; the heat-sensitive recording material containing in the backside layer spherical resin particles having a mean volume particle diameter of 2 to 15 μm in an amount of 0.2 to 5.0 mass % of the backside layer.

This application is a 371 of international applicationPCT/JP2003/011403, which claims priority based on Japanese patentapplication Nos. 2002-267593, 2002-305559 and 2003-75368, filed Sep. 13,2002, Oct. 21, 2002, and Mar. 19, 2003, respectively, which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a heat-sensitive recording materialthat takes advantage of a color forming reaction between anelectron-donating compound and an electron-accepting compound.

BACKGROUND ART

Heat-sensitive recording materials that take advantage of a colorforming reaction between an electron-donating compound and anelectron-accepting compound are relatively inexpensive. Recordingdevices for such heat-sensitive recording materials are compact andtheir maintenance is easy. Therefore, such heat-sensitive recordingmaterials have been used in a broad range of technical fields, forexample, as recording media for facsimiles, word processors, computers,video cassette recorders, medical images and other applications.

Recently, there has been an increasing demand for the development ofheat-sensitive recording materials having excellent transparency andimage quality for use as substitute recording media for silver halidefilms for recoding medical images, typically radiographs for medicalimages. However, heat-sensitive recording materials, in which aheat-sensitive recording layer has been formed on a transparent film toenhance transparency and image quality, suffer blocking, when exposed tohigh humidities, due to adhesion of the front side and backside thereof,particularly when they are used in the form of a roll.

Such a heat-sensitive recording material in which a heat-sensitiverecording layer is formed on a transparent film is disclosed in Japanesepatent No. 2761985, in which a heat-sensitive recording materialcomprises a heat-sensitive recording layer on one side of thetransparent film, and an antireflective layer containing a binder and apigment with a particle diameter of 7.5 to 50 μm on the other side. Theobject of this patent is to enhance the image quality by providing suchantireflective layer to thereby reduce glitter that occurs when theheat-sensitive recording material is viewed through its support, and thepatent does not disclose the problem of preventing blocking or a meansfor solving the problem.

Also known is a heat-sensitive recording material which comprises, onone side of a transparent film, a heat-sensitive recording layer and aprotective layer mainly containing a resin and a filler and formed onthe heat-sensitive recording layer, and, on the other side of thetransparent film, an antistatic layer containing a binder, finespherical resin particles having a particle diameter of about 1 to about6 μm and an antistatic agent (Japanese Unexamined Patent Publication No.1998-193796). This patent publication describes that, due to the use ofthe fine spherical resin particles and the antistatic agent such as aconductive metal oxide in the antistatic layer, this heat-sensitiverecording material allows smooth feeding within recording devices, formsdimensionally accurate images, prevents dust accumulation, and preventsblocking. However, such heat-sensitive recording material, when exposedto high humidities, sometimes suffers blocking because of adhesion ofthe front side and backside.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a heat-sensitiverecording material that does not undergo blocking caused by adhesion ofthe front side and backside thereof even when exposed to highly humidconditions, for example, 40° C. and 90% RH.

In a heat-sensitive recording material which comprises, on one side of atransparent film (hereinafter referred to as the “front side”), aheat-sensitive recording layer containing an electron-donating compound,an electron-accepting compound and a binder, and a protective layercontaining a water soluble resin and/or water dispersible resin(hereinafter collectively referred to as “aqueous resins”), and on theother side of the transparent film (hereinafter referred to as the“backside”) a backside layer containing a pigment and a binder, thepresent invention is characterized in that as a means for solving theproblem described above, spherical resin particles having a mean volumeparticle diameter of 2 to 15 μm are contained as the pigment in thebackside layer in a proportion of 0.2 to 5.0 mass % of the backsidelayer.

In particular, the present invention provides the followingheatsensitive recording materials:

-   Item 1. A heat-sensitive recording material comprising:

(a) a transparent film;

(b) a heat-sensitive recording layer formed on one side of thetransparent film, and containing an electron-donating compound, anelectron-accepting compound, and a binder;

(c) a protective layer formed on the heat-sensitive recording layer, andcontaining an aqueous resin as a main ingredient; and

(d) a backside layer formed on the other side of the transparent film,and containing a pigment and a binder,

the heat-sensitive recording material containing, as the pigment, in thebackside layer spherical resin particles having a mean volume particlediameter of 2 to 15 μm in an amount of 0.2 to 5.0 mass % of the backsidelayer.

-   Item 2. A heat-sensitive recording material according to Item 1,    wherein the average thickness of the backside layer is 0.5 to 10 μm    and is less than the mean volume particle diameter of the spherical    resin particles contained in the backside layer.-   Item 3. A heat-sensitive recording material according to Item 1,    wherein the binder in the backside layer has a glass transition    temperature of 180 to 250° C.-   Item 4. A heat-sensitive recording material according to Item 1,    wherein the binder in the backside layer is a (meth)    acrylamide-based resin binder having a glass transition temperature    of 180 to 250° C.-   Item 5. A heat-sensitive recording material according to Item 4,    wherein the binder in the backside layer further contains an    ionomeric urethane-based resin.-   Item 6. A heat-sensitive recording material according to Item 1,    wherein the aqueous resin in the protective layer is an    acetoacetyl-modified polyvinyl alcohol having a polymerization    degree of 1500 to 3000 and a saponification degree of at least 95    mol %.-   Item 7. A heat-sensitive recording material according to Item 6,    wherein the protective layer further contains an ionomeric    urethane-based resin as another aqueous resin.-   Item 8. A heat-sensitive recording material according to Item 7,    wherein the ionomeric urethane-based resin is present in an amount    of 10 to 60 mass % relative to the acetoacetyl-modified polyvinyl    alcohol.-   Item 9. A heat-sensitive recording material according to Item 1,    wherein the protective layer further contains a fluorine-containing    surfactant and at least one compound selected from the group    consisting of alkylphosphate salts, waxes and higher fatty acid    amides.-   Item 10. A heat-sensitive recording material according to Item 9,    wherein the total amount of said fluorine-containing surfactant and    said at least one compound selected from the group consisting of    alkyl phosphate salts, waxes and higher fatty acid amides is 0.5 to    15 mass % of the protective layer.-   Item 11. A heat-sensitive recording material according to Item 9,    wherein said at least one compound selected from the group    consisting of alkyl phosphate salts, waxes and higher fatty acid    amides is present in a proportion of 50 to 800 mass % relative to    the fluorine-containing surfactant.-   Item 12. A heat-sensitive recording material according to Item 1,    wherein the protective layer contains an alkyl phosphate salt, a    fluorine-containing surfactant, and a compound selected from the    group consisting of waxes and higher fatty acid amides.-   Item 13. A heat-sensitive recording material according to Item 1,    wherein the protective layer contains an alkyl phosphate salt, a    higher fatty acid amide and a fluorine-containing surfactant.-   Item 14. A heat-sensitive recording material according to Item 1,    wherein the binder in the heat-sensitive recording layer contains an    ionomeric urethane-based resin and a styrene-butadiene-based resin.-   Item 15. A heat-sensitive recording material according to Item 14,    wherein the styrene-butadiene-based resin is present in a proportion    of 100 to 300 mass parts per 100 mass parts of the ionomeric    urethane-based resin.-   Item 16. A heat-sensitive recording material according to Item 1,    wherein the electron-donating compound in the heat-sensitive    recording layer is a leuco dye, and the leuco dye is    microencapsulated in a resin film or is in the form of resin    composite particles containing the leuco dye.-   Item 17. A heat-sensitive recording material according to Item 14,    wherein the heat-sensitive recording layer has a thickness of 15 to    30 μm.-   Item 18. A heat-sensitive recording material according to Item 1,    wherein the transparent film is a polyethylene terephthalate film    having a thickness of 40 to 250 μm.-   Item 19. A heat-sensitive recording material according to Item 1    having a haze value of 10 to 50%.

DETAILED DESCRIPTION OF THE INVENTION

Transparent Film

Examples of the transparent film are unstretched or biaxially stretchedpolyethylene terephthalate films, polystyrene films, polypropylenefilms, polycarbonate films, etc. Although the thickness of such filmscan be suitably selected from a broad range, it is preferably about 40to about 250 μm considering ease of application of the backside layercoating composition and the heat-sensitive recording layer coatingcomposition.

Such transparent films may be colored, for example, blue, insofar as thehaze value thereof is not higher than 10% to enhance their suitabilityfor Schaukasten (a view box used when physicians look at X-rayphotographs).

The haze value of heat-sensitive recording materials is preferably about10 to about 50%, and particularly preferably about 10 to about 35%. Thehaze value of the heat-sensitive recording material can be controlled tobe within the aforementioned ranges by suitably selecting the componentsof the backside layer, heat-sensitive recording layer, and protectivelayer; coating amounts of these layers and the like in light of theteaching of this specification.

Backside Layer

According to the present invention, a heat-sensitive recording layer anda protective layer are formed on one side (front side) of thetransparent film, and a backside layer containing a pigment and a binderis formed on the other side (backside) of the transparent film, in whichspherical resin particles having a mean volume particle diameter of 2 to15 μm are contained as the pigment in a proportion of preferably 0.2 to5.0 mass %, more preferably about 0.3 to 3.5 mass %, of the backsidelayer, thereby producing a heat-sensitive recording material that doesnot undergo blocking caused by adhesion of the front side and backsideeven when exposed to conditions of 40° C. and 90% RH.

<Pigments for Backside Layer>

When the proportion of spherical resin particles having a mean volumeparticle diameter of 2 to 15 μm is less than 0.2 mass %, the effect ofpreventing blocking may be significantly impaired, allowing adhesion ofthe front side and backside. When the proportion exceeds 5.0 mass %, thehaze value of the heat-sensitive recording material may become low.

Moreover, when spherical resin particles having a mean volume particlediameter of less than 2 μm are used, the effect of preventing blockingmaybe significantly impaired. On the other hand, when spherical resinparticles having a mean volume particle diameter more than 15 μm areused, the resin particles may easily separate from the backside layer,or the front side of the heat-sensitive recording material may bedamaged. The more preferable mean volume particle diameter is about 3 toabout 10 μm.

In this specification, the “mean volume particle diameter” of sphericalresin particles is measured according to the Coulter counter method.

Absolutely spherical resin particles are preferably used in the backsidelayer. However, resin particles that are not absolutely spherical arealso usable. Although the sphericity thereof is not limited, asphericity of 0.7 or greater is preferable. Sphericity herein refers tothe ratio of the minor axis (X) to the major axis (Y) of a resinparticle (X/Y).

Spherical resin particles are preferably made of, for example,acryl-based resins, styrene-based resins, silicone-based resins,polycarbonate-based resins, etc. Among such examples, acryl-based resinsand styrene-based resins are preferable due to their cost advantages. Inparticular, acryl-based resins, especially methyl methacrylate resins,are preferable due to their cost advantages and strength.

The aforementioned spherical resin particles are known and are readilyavailable, and a variety of such resin particles are commerciallyavailable.

<Binders for Backside Layer>

Examples of binders usable in the backside layer are casein, polyvinylalcohol-based resins, diisobutylene-maleic anhydride-based resins,styrene-maleic anhydride-based resins, acryl-based resins (e.g., acrylicacid-acrylic acid ester copolymer latexes and the like),(meth)acrylamide-based resins, vinyl acetate-based resins,urethane-based resins, etc.

In the description and claims, the term “(meth)acrylamide” is intendedto mean at least one member selected from the group consisting ofmethacrylamide and acrylamide.

The glass transition temperature (Tg) of such binders is not limited andmaybe suitably selected from a broad range. Generally, however, theglass transition temperature is 180 to 250° C., more preferably 200 to230° C.

In particular, the use of a (meth)acrylamide-based resin binder having aglass transition temperature of 180 to 250° C., particularly 200 to 230°C., produces the effect of inhibiting curling inward in the direction ofthe recording layer in a low-humidity environment both before and afterrecording. Among such (meth)acrylamide-based resins, preferable arecore-shell-structured latexes in which, for example, the shell is madeof a (meth)acrylamide-based resin and the core is made of an acrylicacid ester-based resin (weight ratio of core:shell=1:1 to 5). Suchcore-shell-structured latexes are known, and are disclosed in, forexample, Japanese Unexamined Patent Publication No. 1993-69665, and arealso commercially available.

The resin constituting the shell of the resin particles having theaforementioned core/shell structure is produced by seed-polymerizing atleast one monomer in the presence of an aqueous dispersion of seedparticles. As the resin of the shell, a resin prepared byseed-polymerizing at least one member selected from the group consistingof methacrylamide and acrylamide is particularly preferable.

Such a resin can be obtained according to known methods, for example, amethod disclosed in Japanese Unexamined Patent Publication No.1993-69665, by emulsion-polymerizing at least one member selected fromthe group consisting of methacrylamide and acrylamide using, as cores,hydrophobic particles (seed particles) produced by polymerizing one ormore unsaturated monomers.

If necessary, in the seed-polymerization, (meth)acrylamide may beconjointly used with one or more other unsaturated monomerscopolymerizable with (meth)acrylamide. Examples of such otherunsaturated monomers are methyl(meth)acrylate, ethyl(meth)acrylate,butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate,2-aminoethyl(meth)acrylate, glycidyl (meth)acrylate, (meth)acrylic acid,maleic anhydride, itaconic acid, fumaric acid, crotonic acid,(meth)acrylonitrile, styrene, α-methylstyrene, divinylbenzene, etc.

The proportion of (meth)acrylamide contained in the seed-polymerizedresin(s) constituting the shell is 50 to 100 mass %, and preferably 70to 100 mass %, of the seed-polymerized resin(s) constituting the shell.

Examples of seed particles include various known latex particles ofacrylic acid ester-based latexes such as methyl(meth)acrylate,ethyl(meth)acrylate or butyl(meth)acrylate; styrene-butadiene-basedlatexes; styrene-acrylate-based latexes; etc. Copolymerized(meth)acrylamide may be present in the seed particles.

In the present invention, when a core/shell-structured resin asdescribed above is used, the glass transition temperature of the binderused in the backside layer refers to the glass transition temperature ofthe resin constituting the shell.

The proportion of the binder having a glass transition temperature of180 to 250° C. is preferably about 30 to about 99.8 mass %, andparticularly preferably about 50 to about 80 mass %, of the total solidscontent of the backside layer.

If necessary, adhesion of the backside layer to the transparent filmsupport may be enhanced by using a urethane-based resin binder,particularly an ionomeric urethane-based resin, in the backside layer ina proportion of about 3 to about 30 mass %, particularly about 5 toabout 20 mass %, of all the binders.

Such ionomeric urethane-based resins to be used include, for example,those disclosed in Japanese Unexamined Patent Publication No. 1993-8542(paragraphs 0017 to 0019 in particular). Unlike conventionalemulsion-type resins in which a polyurethane resin is dispersed in waterwith the use of an emulsifier or the like, such ionomeric urethane-basedresins are aqueous urethane resins in which a polyurethane resin havingionic nature, namely ionomeric urethane-based resin, due to its ionicgroups, is dissolved or colloidally dispersed in the form of very fineparticles in water without the use of an emulsifier or an organicsolvent. Typical examples of ionomeric urethane-based resins are resinsof Hydran HW series and Hydran AP series manufactured by DAINIPPON INKAND CHEMICALS INC., resins of Superflex series manufactured by DAI-ICHIKOGYO SEIYAKU CO., LTD., and the like.

It is preferable that the proportion of the binder in the backside layeris about 80 to about 99.8 mass %, particularly about 90 to about 99.5mass %, of the total solids of the backside layer.

<Method for Forming Backside Layer and Other Particulars>

The backside layer can be formed by preparing a backside layer coatingcomposition, using water as a medium, by stirring and mixing sphericalresin particles having a mean volume particle diameter of 2 to 15 μm, abinder and, if desired additives that can be contained in theheat-sensitive recording layer described below, applying the backsidelayer coating composition to the backside of a transparent film, anddrying the resulting layer.

Although the thickness of the backside layer is not limited, the averagethickness of the backside layer is preferably about 0.5 to about 10 μm,and more preferably about 2 to about 6 μm. The backside layer having anaverage thickness less than the mean volume particle diameter of thespherical resin particles contained in the backside layer produces theeffect of reducing the frictional resistance between the front side andthe backside of a heat-sensitive recording material, thereby inhibitingmulti-feeding problem in printers for sheet-form heat-sensitiverecording materials (i.e., the problem that two or more sheets of asheet-form heat-sensitive recording material are simultaneously fed).The average thickness of the backside layer herein refers to thatmeasured by electron microscope.

Although the backside layer coating composition is usually applied in anamount of 0.1 to 15 g/m², and in particular 0.5 to 10 g/m², on dryweight basis, it is preferable to apply the composition in an amountsuch that the average thickness of the backside layer is 0.5 to 10 μmfrom the standpoint of inhibiting multi-feeding problem as describedabove.

Due to the provision of the specific backside layer, the heat-sensitiverecording material of the present invention exhibits excellent blockingresistance and pre- and post-recording curl resistances. To furtherenhance post-recording curl resistance, the heat-sensitive recordingmaterial can be subjected to a reverse curl treatment. The reverse curltreatment is intended to mean a treatment comprising winding, after theformation of the respective layers, the resulting heat-sensitiverecording material with the protective layer facing outward, and curingthe heat-sensitive recording material in this position, thereby giving acurl to the backside. If it is desired to precisely control the extentof the reverse curling, the heat-sensitive recording material that hasbeen cut in the form of a sheet may be subjected to a curing treatmentwhile it is kept reverse-curled using a curled metal plate or the like.Although curing treatment can be carried out under a variety ofconditions, the heat-sensitive recording material is preferably cured byallowing it to stand at, for example, 30 to 50° C. and 20 to 80% RH for1 to 5 days.

Heat-Sensitive Recording Layer

For the thermal recording method taking advantage of a combination of anelectron-donating compound and an electron-accepting compound containedin the heat-sensitive recording layer, examples of such a combinationare a combination of a leuco dye and a developer, a combination of adiazonium salt and a coupler, a combination of an organic silver saltand a reducing agent, a combination of a transition element such asiron, cobalt, copper or the like with a chelating compound, acombination of an aromatic isocyanate compound and an imino compound,and the like. The combination of a leuco dye and a developer ispreferably used because it gives excellent color density. Hereinbelow, aheat-sensitive recording material employing a combination of a leuco dyeand a developer will be described in detail.

<Leuco Dyes and Developers>

A wide variety of known leuco dyes and developers are usable. Specificexamples of leuco dyes are

-   3-[2,2-bis(1-ethyl-2-methylindol-3-yl)vinyl]-3-(4-diethylamino-phenyl)phthalide,    3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide,    3-(4-diethylamino-2-methylphenyl)-3-(4-diethylaminophenyl)-6-dimethylaminophthalide,    3-cyclohexylamino-6-chlorofluoran,    3-diethylamino-6-methyl-7-chlorofluoran,    3-diethylamino-6,8-dimethylfluoran, 3-diethylamino-7-chlorofluoran,    3-(N-ethyl-N-isoamyl)amino-6-methyl-7-anilinofluoran,    3-di(n-butyl)amino-6-methyl-7-anilinofluoran,    3-di(n-pentyl)amino-6-methyl-7-anilinofluoran,    3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluoran,    3-di(n-butyl)amino-6-chloro-7-anilinofluoran,    3-pyrrolidino-6-methyl-7-anilinofluoran,    3-piperidino-6-methyl-7-anilinofluoran,    3,3-bis[1-(4-methoxyphenyl)-1-(4-dimethylaminophenyl)ethylen-2-yl]-4,5,6,7-tetrachlorophthalide,    3-p-(p-dimethylaminoanilino)anilino-6-methyl-3-p-(p-chloroanilino)anilino-6-methyl-7-chlorofluoran,    3-[1,1-bis(1-ethyl-2-methylindol-3-yl)]-3-p-diethylaminophenyl-phthalide,    3,3-bis(1-n-butyl-2-methylindol-3-yl)phthalide,    3,6-bis(dimethylamino)fluorene-9-spiro-3′-(6′-dimethylamino)-phthalide,    3,3-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide, etc.

Leuco dyes are not limited to those given above. Leuco dyes are usablein combination of two or more species. Although the amount of leuco dyecannot be specified because it varies depending on the developer to beused, it is preferably about 5 to about 35 mass %, and particularlypreferably about 8 to about 25 mass %, of the total solids content ofthe heat-sensitive recording layer.

Examples of developers are 4,4′-isopropylidenediphenol,

-   4,4′-cyclohexylidenediphenol, 1,1-bis(4-hydroxyphenyl)ethane,    1,1-bis(4-hydroxyphenyl)-1-phenylethane,    4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone,    4-hydroxy-4′-isopropoxydiphenylsulfone,    3,3′-diallyl-4,4′-dihydroxydiphenylsulfone,    2,2′-bis[4-(4-hydroxyphenyl)phenoxy]diethylether,    4,4′-bis[(4-methyl-3-phenoxycarbonylaminophenyl)ureide]diphenyl-sulfone,    N-p-toluenesulfonyl-N′-3-(p-toluenesulfonyloxy)phenylurea,    3,3′-bis(p-toluenesulfonylaminocarbonylamino)diphenylsulfone, benzyl    4-hydroxybenzoate, N,N′-di-m-chlorophenylthiourea,    N-p-tolylsulfonyl-N′-phenylurea,    4-4′-bis(p-tolylsulfonylaminocarbonylamino)diphenylmethane, zinc    4-[2-(p-methoxyphenoxy)ethyloxy]salicylate, zinc    4-[3-(p-tolylsulfonyl)propyloxy]salicylate, zinc    5-[p-(2-p-methoxyphenoxyethoxy)cumyl]salicylate, etc.

The ratio of the developer to the leuco dye may be suitably selectedaccording to the type of the leuco dye and the developer and is notparticularly limited. Generally, however, the developer is used in anamount of about 1 to about 10 mass parts, preferably about 2 to about 6mass parts, per mass part of the leuco dye.

The use of a leuco dye microencapsulated in a resin film or in the formof resin composite particles containing the dye gives a heat-sensitiverecording materials having a low haze value, and is thereforepreferable. The mean volume particle diameter of such microcapsules andcomposite particles is preferably about 0.5 to about 3.0 μm, andparticularly preferably about 0.5 to about 2.0 μm. Microencapsulatedleuco dyes are known, and are disclosed in, for example, U.S. Pat. No.4,682,194. Composite particles in which a leuco dye is contained in aresin are also known, and are disclosed in, for example, U.S. Pat. No.5,804,528. The disclosures of these U.S. patents are incorporated hereinby reference.

Particularly preferable composite particles are those comprising a leucodye and a polyurea or polyurea-polyurethane resin. Preferable suchcomposite particles are described below.

Composite particles comprising a leuco dye and a polyurea orpolyurea-polyurethane resin is obtained by, for example, emulsifying anddispersing an oily solution containing a polyisocyanate compound and aleuco dye as dissolved therein, in a solution of a hydrophilicprotective colloid such as polyvinyl alcohol to a mean particle diameterof about 0.5 to about 3 μm, and effecting the polymerization reaction ofthe polyisocyanate compound. The amount of the leuco dye contained inthe composite particles is about 5 to about 70 mass %, and preferablyabout 30 to about 60 mass %, of the composite particles.

The specific leuco dye contained in the composite particles gives theeffect of enhancing the transparency of the heat-sensitive recordinglayer compared with the use of the specific leuco dye alone in the formof a particle, presumably because the specific leuco dye in thecomposite particles is highly isolated from outside, so that backgroundfogging and the disappearance of developed images due to heat orhumidity therefore substantially do not occur, and the specific leucodye is uniformly mixed with the resin component of the compositeparticles.

The polyisocyanate compound reacts with water to form an amine compound.This amine compound reacts with a polyisocyanate compound to formpolyurea. These reactions and a reaction between an organic compoundhaving a hydroxyl group and a polyisocyanate compound givepolyurea-polyurethane.

The polyisocyanate compound may be used singly, or in the form of amixture with at least one member selected from the group consisting ofpolyols and polyamines that can react with the polyisocyanate compound,or in the form of a polyisocyanate-polyol adduct or a multimer such as abiuret or an isocyanurate.

The specific leuco dye is dissolved in such a polyisocyanate compound.The solution is emulsified and dispersed in an aqueous medium containinga protective colloid substance such as polyvinyl alcohol as dissolvedtherein, and if necessary a reactive substance such as a polyamine isadded thereto. The resulting emulsion or dispersion is then heated topolymerize the polymer-forming ingredients, thereby forming compositeparticles comprising the specific leuco dye and the resulting polymericsubstance.

Examples of the polyisocyanate compound are p-phenylene diisocyanate,1,3-bis(1-isocyanato-1-methylethyl)benzene, 2,6-tolylene diisocyanate,2,4-tolylene diisocyanate, naphthalene-1,4-diisocyanate,dicyclohexylmethane-4,4′-diisocyanate,1,3-bis(isocyanatomethyl)cyclohexane,3,3′-dimethyldiphenylmethane-4,4′-diisocyanate,xylylene-1,4-diisocyanate, 4,4′-diphenylpropane diisocyanate,hexamethylene diisocyanate, butylene-1,2-diisocyanate,cyclohexylene-1,2-diisocyanate, cyclohexylene-1,4-diisocyanate,4,4′,4″-triphenylmethane triisocyanate, toluene-2,4,6-triisocyanate,trimethylolpropane adduct of hexamethylene diisocyanate,trimethylolpropane adduct of 2,4-tolylene diisocyanate,trimethylolpropane adduct of xylylene diisocyanate, etc.

Examples of polyol compounds are ethylene glycol, 1,3-propanediol,1,4-butanediol, 1,7-heptanediol, 1,3-octanediol, propylene glycol,1,3-dihydroxybutane, 2,2-dimethyl-l, 3-propanediol, 2,5-hexanediol,3-methyl-1,5-pentanediol, 1,4-cyclohexanedimethanol,dihydroxycyclohexane, diethylene glycol, phenylethylene glycol,pentaerythritol, 1,4-di(2-hydroxyethoxy)benzene, 1,3-di(2-hydroxyethoxy)benzene, p-xylylene glycol, m-xylylene glycol,4,4′-isopropylidenediphenol, 4,4′-dihydroxydiphenylsulfone, etc.

Examples of the polyamine compound are ethylenediamine,trimethylenediamine, tetramethylenediamine, pentamethylenediamine,hexamethylenediamine, p-phenylenediamine, 2,5-dimethylpiperazine,triethylenetriamine, triethylenetetramine, diethylaminopropylamine,tetraethylenepentamine, pentaethylenehexamine, etc.

The above polyisocyanate compounds, polyamines, polyol adducts ofpolyisocyanates, polyol compounds, etc., are not limited to those givenabove, and can be used in a combination of two or more, if so desired.

<Print Stability-Improving Agents and Sensitizers>

The heat-sensitive recording layer may contain a printstability-improving agent to enhance the long-term stability of recordedportions and a sensitizer to optimize the recording sensitivity.Examples of such print stability-improving agents are hindered-phenolcompounds such as

-   2,2′-ethylidenebis(4,6-di-tert-butylphenol),    4,4′-thiobis(2-methyl-6-tert-butylphenol),    1,3,5-tris-(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanuric    acid, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,    1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane,    2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane, and the like; epoxy    compounds such as 1,4-diglycidyloxybenzene,    4,4′-diglycidyloxydiphenylsulfone,    4-benzyloxy-4′-(2-methylglycidyloxy)diphenylsulfone, diglycidyl    terephthalate, cresol novolac epoxy resins, phenol novolac epoxy    resins, bisphenol A epoxy resins, and the like;    N,N′-di-2-naphthyl-p-phenylenediamine; sodium salt or polyvalent    metal salts of-   2,2′-methylenebis(4,6-di-tert-butylphenyl)phosphate;    bis(4-ethyleneiminocarbonylaminophenyl)methane; etc.

Examples of the sensitizer are stearamide, methylenebisstearamide,dibenzyl terephthalate, benzyl p-benzyloxybenzoate, 2-naphthyl benzylether, m-terphenyl, p-benzylbiphenyl, p-tolyl biphenyl ether,di(p-methoxyphenoxyethyl)ether, 1,2-di(3-methylphenoxy)ethane,1,2-di(4-methylphenoxy)ethane, 1,2-di(4-methoxyphenoxy)ethane,1,2-di(4-chlorophenoxy)ethane, 1,2-diphenoxyethane,1-(4-methoxyphenoxy)-2-(3-methylphenoxy)ethane, p-methylthiophenylbenzyl ether, 1,4-di(phenylthio)butane, p-acetotoluidide,p-acetophenetidide, N-acetoacetyl-p-toluidine,di(β-biphenylethoxy)benzene, oxalic acid di-p-chlorobenzyl ester, oxalicacid di-p-methylbenzyl ester, oxalic acid dibenzyl ester, etc.

Although the proportion of the print stability-improving agent is notlimited, it is usually used in an amount of about 0.01 to about 4 massparts per mass part of the developer. Although the amount of sensitizeris not limited, it is usually used in an amount of about 0.01 to about 4mass parts per mass part of the developer.

<Binders for Heat-Sensitive Recording Layer>

Examples of binders usable for forming the heat-sensitive recordinglayer include, for example, water-soluble binders such as starches,hydroxyethylcellulose, methylcellulose, carboxymethylcellulose, casein,polyvinyl alcohol, carboxyl modified polyvinyl alcohol,diacetoneacrylamide-modified polyvinyl alcohol, acetoacetyl-modifiedpolyvinyl alcohol, silicon-modified polyvinyl alcohol,diisobutylene-maleic anhydride copolymers, styrene-maleic anhydridecopolymers, ethylene-acrylic acid copolymers, styrene-acrylic acidcopolymers, and the like; water-dispersible binders such as vinylacetate-based resins, styrene-butadiene-based resins, acryl-basedresins, urethane-based resins, and the like.

Among them, the use of a urethane-based resin in combination with astyrene-butadiene-based resin is preferable. In particular, the use ofan ionomeric urethane-based resin in combination with astyrene-butadiene-based resin affords the effect of preventing blurringof recorded image edges even when the recording energy is increasedduring thermal head recording.

Usually, recorded image edges are likely to be blurred by increasedrecording energy when a transparent film is used as the support and thethickness of the heat-sensitive recording layer exceeds 10 μm. However,the use of an ionomeric urethane-based resin in combination with astyrene-butadiene-based resin produces the effect of preventing recordedimage edges from blurring and imparting excellent gradation of recordedimages even when the heat-sensitive recording layer has a thickness of15 to 30 μm.

Examples of such ionomeric urethane-based resins are those that can beused in the aforementioned backside layer as a binder.

Although the proportion of ionomeric urethane-based resin tostyrene-butadiene-based resin is not limited, preferably thestyrene-butadiene-based resin is used in an amount of about 100 to about300 mass parts, and particularly about 100 to about 200 mass parts, per100 mass parts of the ionomeric urethane-based resin.

The amount of the binder, in particular the total amount of theionomeric urethane-based resin and the styrene-butadiene-based resin, inthe heat-sensitive recording layer is about 10 to about 40 mass %, andpreferably about 15 to about 35 mass %, of the heat-sensitive recordinglayer.

Ionomeric urethane-based resins and styrene-butadiene-based resins areeach used in the form of a latex.

<Additives>

A variety of additives may be used in the heat-sensitive recordinglayer. Examples of such additives are pigments such as amorphous silica,calcium carbonate, zinc oxide, aluminum oxide, titanium dioxide,aluminum hydroxide, barium sulfate, talc, kaolin, clay, calcined kaolinor urea-formaldehyde resin fillers, in which the primary particlesthereof have a mean particle diameter of about 0.01 to about 2.0 μm;surfactants such as sodium dioctylsulfosuccinate, sodiumdodecylbenzenesulfosuccinate, sodium lauryl sulfate, fatty acid metalsalts and the like; lubricants; antifoaming agents; thickeners;pH-adjusters; ultraviolet absorbers; light stabilizers; crosslinkingagents; fluorescent dyes; coloring dyes; etc. Additives are not limitedto those given above, and they can be used in a combination of two ormore.

<Method for Forming Heat-Sensitive Recording Layer>

The heat-sensitive recording layer can be formed by, for example,concurrently or separately pulverizing a leuco dye, a developer, and ifdesired, a sensitizer, a print stability-improving agent and the like bymeans of a ball mill, attritor, sand mill or like mixing/pulverizingapparatus to a mean particle diameter of 3 μm or less, and preferably 2μm or less; adding at least a binder thereto to prepare a heat-sensitiverecording layer coating composition; applying the coating composition tothe front side of the transparent film in an amount such that thethickness thereof after drying is, for example, about 3 to about 35 μm,and preferably about 15 to about 30 μm; and drying the coatingcomposition. It is usually sufficient that the heat-sensitive recordinglayer coating composition is applied to the front side of thetransparent film in an amount such that the amount thereof after dryingis about 3 to about 35 g/m², and preferably about 15 to about 30 g/m²,and the resulting coating is then dried.

Protective Layer

A protective layer that mainly contains an aqueous film-forming resin isprovided on the heat-sensitive recording layer to enhance runnabilityduring recording, friction-fogging resistance, chemical resistance, andresistance to blocking with the backside layer of the heat-sensitiverecording material. This produces the effect of increasing thetransparency of the heat-sensitive recording material.

Examples of such an aqueous resin in the protective layer are, forexample, said at least one member selected from the group consisting ofwater-soluble resins and water-dispersible resins that are usable asbinders in the aforementioned heat-sensitive recording layer.

Although the amount of said at least one member selected from the groupconsisting of water-soluble resins and water-dispersible resins can beselected from a broad range, it is usually about 40 to about 95 mass %,and preferably about 50 to about 80 mass %, of the protective layer.

Among such aqueous resins, acetoacetyl-modified polyvinyl alcohol havinga polymerization degree of 1500 to 3000 and a saponification degree of95 mol % or greater (hereinafter referred to as “specificacetoacetyl-modified polyvinyl alcohol”) is preferable to increaseresistance to blocking with the backside layer.

When the polymerization degree of the acetoacetyl-modified polyvinylalcohol is less than 1500, thermal head recording at an increasedrecording energy is likely to give rough-surfaced recorded portion,thereby impairing suitability for Schaukasten. When the polymerizationdegree exceeds 3000, the concentration of the protective layer coatingcomposition has to be lowered in order to adjust the viscosity of theprotective layer coating composition to be in the applicable range forforming the protective layer, and therefore the resulting protectivelayer coating composition is likely to become less easy to apply andfail to produce a uniform protective layer surface.

A particularly preferable polymerization degree of the specificacetoacetyl-modified polyvinyl alcohol is about 2100 to about 2500.

When the saponification degree of acetoacetyl-modified polyvinyl alcoholis less than 95 mol %, the protective layer is likely to stick to thethermal head during recording to impair the recorded image quality.

Moreover, when the saponification degree of the specific polyvinylalcohol is less than 95 mol %, and if an ionomeric urethane-based resinis also used as an aqueous resin to enhance the water resistance of theprotective layer, the surface of the protective layer becomes cloudy andthe transparency of the heat-sensitive recording material is therebylowered, presumably due to the low compatibility between the specificpolyvinyl alcohol and the ionomeric urethane-based resin, resulting inimpaired Schaukasten suitability.

It is preferable that the specific polyvinyl alcohol has an acetoacetylmodification degree of preferably about 0.5 to about 10 mol %. Anacetoacetyl modification degree of less than 0.5 mol % is likely toimpair water resistance. An acetoacetyl modification degree exceeding 10mol % is likely to impair not only the water solubility of theacetoacetyl-modified polyvinyl alcohol itself but also the waterresistance of the protective layer.

If desired, in addition to the aforementioned specific polyvinylalcohol, an ionomeric urethane-based resin may be used as an aqueousresin in the protective layer, thereby affording the effect ofincreasing sticking resistance during recording and increasing waterresistance of the protective layer. Examples of such ionomeric urethaneresins are those that are usable in the aforementioned backside layer asa binder.

The ionomeric urethane-based resin in the protective layer is preferablyused in an amount of about 10 to about 60 mass %, more preferably about20 to about 50 mass %, relative to the specific acetoacetyl-modifiedpolyvinyl alcohol. When the amount of the ionomeric urethane-based resinis less than 10 mass % relative to the specific acetoacetyl modifiedpolyvinyl alcohol, the effect of enhancing water resistance may beinsufficient. When the amount exceeds 60 mass %, the chemical resistanceof recorded portions may be impaired.

Addition, to the heat-sensitive recording layer, of a crosslinking agentthat crosslinks to the specific acetoacetyl-modified polyvinyl alcoholin the protective layer, enhances the film forming ability of theprotective layer to be formed on the heat-sensitive recording layer,adhesion between the protective layer and the heat-sensitive recordinglayer, and water resistance of the protective layer. Examples of suchcrosslinking agents are glyoxal, adipic dihydrazide, dimethylolurea,dialdehyde starches, melamine resins, polyamidoamine-epichlorohydrinresins, borax, boric acid, ammonium zirconium carbonate, etc. The amountof crosslinking agent is preferably about 1 to about 20 mass parts, andparticularly about 2 to about 15 mass parts, per 100 mass parts of thespecific acetoacetyl modified polyvinyl alcohol in the protective layer.

The protective layer may further contain, for example, pigments such ascalcium carbonate, zinc oxide, aluminum oxide, titanium dioxide,amorphous silica, aluminum hydroxide, barium sulfate, talc, kaolin,styrene resin fillers, nylon resin fillers, urea-formaldehyde resinfillers and the like; lubricants such as zinc stearate, calcium stearateand the like; waxes such as paraffin, polyethylene wax, polypropylenewax, carnauba wax and the like; surfactants such as perfluoroalkylcarboxylic acid salts, perfluoroalkyl phosphate salts, perfluoroalkylsulfonate salts, ethylene oxide adducts of perfluoroalkyl amide, dialkylsulfosuccinate salts, alkylsulfonic acid salts, alkyl carboxylic acidsalts, alkyl phosphate salts, alkyl ethylene oxides, and the like; andauxiliaries such as higher fatty acid amide including stearamide,methylenebisstearamide, ethylenebisstearamide and the like.

Particularly, the use of a fluorine-containing surfactant in combinationwith at least one member selected from alkyl phosphate salts, waxes andhigher fatty acid amides affords excellent sticking resistance and theeffect of preventing the impairment of recorded image quality caused bythe residual substance accumulation on thermal heads.

In particular, it is preferable to use a fluorine-containing surfactantand an alkyl phosphate in combination with a wax or a higher fatty acidamide. Especially, combined use of a fluorine-containing surfactant, analkyl phosphate salt and a higher fatty acid amide is preferable.

With respect to the proportion of a fluorine-containing surfactant tosaid at least one member selected from alkyl phosphate salts, waxes andhigher fatty acid amides, it is preferable to use said at least onemember selected from alkyl phosphate salts, waxes and higher fatty acidamides in an amount of 50 to 800 mass %, and particularly preferably 100to 500 mass %, relative to the fluorine-containing surfactant.

When a fluorine-containing surfactant and an alkyl phosphate salt areused in combination with a wax or a higher fatty acid amide, it ispreferable to use the alkyl phosphate salt in an amount of about 10 toabout 100 mass %, and to use the wax or higher fatty acid amide in anamount of about 50 to about 600 mass %, relative to thefluorine-containing surfactant.

The total amount of the fluorine-containing surfactant and said at leastone member selected from the group consisting of alkyl phosphate salts,waxes and higher fatty acid amides is preferably 0.5 to 15 mass %, andparticularly 3 to 12 mass %, of the protective layer.

Preferable fluorine-containing surfactants are anionic or nonionic ones,and include, for example, perfluoroalkyl carboxylic acid salts,perfluoroalkyl phosphate salts, perfluoroalkylsulfonic acid salts,ethylene oxide adducts of perfluoroalkyl amide, etc. Alkyl groups insuch compounds preferably have about 6 to about 30 carbon atoms.Lithium, potassium and ammonium salts are preferable among such salts.Nonionic ethylene oxide adducts of perfluoroalkyl amide (especially,those in which the number of moles of ethylene oxide added is about 5 toabout 20) are particularly preferable.

Preferable alkyl phosphate salts are, for example, salts of monoalkylphosphates and salts of dialkyl phosphates, the alkyl having about 8 toabout 24 carbon atoms. Lithium, potassium, and ammonium salts arepreferable among such salts. Potassium salts of monoalkyl phosphates areparticularly preferable.

Examples of waxes include paraffin wax, polyethylene wax, polypropylenewax, and the like having a melting point of about 50 to about 120° C.Among them, polyethylene wax is preferable.

Examples of higher fatty acid amides include C₁₆₋₂₄ higher fatty acidamide, such as stearamide, behenamide, ethylenebisstearamide, etc. Amongthem, stearamide is preferable.

The mean volume particle diameter of such waxes and higher fatty acidamides is not particularly limited. Generally, however, it is preferablyabout 0.1 to about 3.0 μm, and more preferably about 0.1 to about 2.0μm.

The protective layer can be formed, typically using water as a medium,by preparing a protective layer coating composition by stirring andmixing the aqueous resin and, if desired, pigments, crosslinking agents,waxes, higher fatty acid amide, surfactants, etc.; applying theprotective layer coating composition to the heat-sensitive recordinglayer in an amount such that the amount thereof after drying is about0.5 to about 10 g/m², and preferably about 1 to about 5 g/m²; and dryingthe coating.

Coating compositions for respective layers can be applied according toany of known coating methods such as a slot-die method, slide beadmethod, curtain method, air knife method, blade method, gravure method,roll coater method, spray method, dip method, bar method, extrusionmethod, and the like.

After forming all the layers, performing a smoothing treatment accordingto a known smoothing method such as supercalendering, softcalendering,etc., is effective for improving recording sensitivity. Theheat-sensitive recording layer and the protective layer may be treatedby being pressed against either the metal roll or the elastic roll ofsuch calendar.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be illustrated in further detail withreference to Examples below. It should be understood that the scope ofthe invention is not limited by these Examples. Herein, “parts” and “%”represent “mass parts” and “mass %”, respectively, unless otherwisespecified. With respect to the spherical resin particles used for thebackside layer, the “mean volume particle diameter” thereof was measuredaccording to the Coulter counter method, and the mean volume particlediameter of particles for other purposes is measured according to thelaser diffraction method, unless otherwise specified.

EXAMPLE 1

Preparation of Backside Layer Coating Composition

A composition containing 425 parts of a core-shell latex in which theshell is made of an acrylamide-based resin (glass transitiontemperature: 218° C.) and the core is made of an acrylic acid esterresin (glass transition temperature: 10° C.) (manufactured by MitsuiChemicals, Inc., Bariastar (registered trademark) B-1000, weight ratioof core: shell=1:1.5, solids content: 20%) and 75 parts of an ionomericurethane-based resin latex (manufactured by Dainippon Ink & Chemicals,Inc., Hydran (registered trademark) AP-30F, solids content: 20%) asbinders, and 0.5 parts of spherical resin particles having a mean volumeparticle diameter of 8 μm (measured according to the Coulter countermethod) (manufactured by Ganz Chemical Co., Ltd., Ganz Pearl (registeredtrademark) GM-0801, polymethylmethacrylate) was stirred to give abackside layer coating composition.

Preparation of Dispersion of Leuco Dye-Containing Composite Particles(Dispersion A)

Leuco dyes (12 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 5parts of 3-diethylamino-6,8-dimethylfluoran, and 3 parts of3,3-bis(4-diethylamino-2-ethoxyphenyl)-4-azaphthalide) and a UV-absorber(5 parts of 2-hydroxy-4-octyloxybenzophenone) were dissolved withheating (150° C.) in a mixed solvent of 11 parts ofdicyclohexylmethane-4,4′-diisocyanate (manufactured by Sumitomo BayerUrethane Co., Ltd., Desmodule W) and 11 parts of m-tetramethylxylylenediisocyanate (manufactured by Mitsui Takeda Chemicals, Inc., TMXDI).This solution was slowly added to 100 parts of an aqueous solutioncontaining 8.8 parts of polyvinyl alcohol (manufactured by Kuraray Co.,Ltd., Kuraray Poval (registered trademark) PVA-217EE) and as asurfactant 0.5 parts of an ethyleneoxide adduct of acetylene glycol(manufactured by Nissin Chemical Industry Co., Ltd., Olfine E1010), andthe resulting mixture was emulsified and dispersed in a homogenizer at10000 rpm.

To this emulsion/dispersion was added 30 parts of water and an aqueoussolution prepared by dissolving 2.5 parts of a polyamine compound(manufactured by Shell International Petroleum Co., Epicure T) in 22.5parts of water to homogenize the emulsion/dispersion. Theemulsion/dispersion was heated to 75° C. to carry out polymerizationreaction for 7 hours, thereby giving a black-color-forming compositeparticle dispersion having a mean volume particle diameter of 0.8 μm(measured according to the laser diffraction method).

The solids content of the black-color-forming composite particlesdispersion was adjusted with water to 20%.

Preparation of Dispersion B

A composition containing 25 parts of 4,4′-dihydroxydiphenylsulfone, 15parts of 3,3′-diallyl-4,4′-dihydroxydiphenylsulfone, 40 parts of a 25%aqueous solution of polyvinyl alcohol (manufactured by Kuraray Co.,Ltd., Kuraray Poval PVA-203), 5 parts of a 2% emulsion of a natural oiland fat-based antifoaming agent, 10 parts of a 5% aqueous solution ofsodium dioctylsulfosuccinate, and 50 parts of water was pulverized by ahorizontal sand mill (manufactured by Aimex Co., Ltd., Ultra Visco MillUVX-2) to a mean volume particle diameter of 0.3 μm (measured accordingto the laser diffraction method), thereby giving Dispersion B.

Preparation of Heat-Sensitive Recording Layer Coating Composition

A composition containing 150 parts of Dispersion A, 115 parts ofDispersion B, 20 parts of a 7% aqueous solution of polyvinyl alcohol(Kuraray Co., Ltd., Kuraray Poval (registered trademark) PVA-235), 30parts of a styrene-butadiene-based latex (manufactured by Nippon A&LInc., solids content: 48%, Smartex (registered trademark) PA9281), 50parts of an ionomeric urethane-based resin latex (manufactured byDainippon Ink & Chemicals, Inc., Hydran (registered trademark) AP-30F,solids content: 20%), 8 parts of a 5% aqueous solution of adipicdihydrazide, and 30 parts of water was stirred to give a heat-sensitiverecording layer coating composition.

Preparation of Protective Layer Coating Composition

A composition containing 100 parts of an ionomeric urethane-based resinlatex (manufactured by Dainippon Ink & Chemicals Inc., Hydran(registered trademark) AP-30F, solids content: 20%), 500 parts of an 8%aqueous solution of an acetoacetyl-modified polyvinyl alcohol(manufactured by Nippon Synthetic Chemical Industry Co., Ltd.,Gohsefimer (registered trademark) OKS-3431, degree of polymerization:about 2300, degree of saponification: about 98 mol %, degree ofacetoacetyl modification: 4 mol %), 5 parts of a 25% aqueous solution ofa polyamidoamine-epichlorohydrin, 50 parts of a 60% slurry of kaolinhaving a mean volume particle diameter of 0.8 μm (manufactured byEngelhard Corporation, UW-90), 26 parts of stearamide (manufactured byChukyo Yushi Co., Ltd., Hymicron L271, solids content: 25%, mean volumeparticle diameter: 0.4 μm), 4 parts of potassium stearyl phosphate(manufactured by Matsumoto Yushi Seiyaku, Woopol (registered trademark)1800, solids content: 35%), 15 parts of a 10% aqueous solution of anethylene oxide adduct of perfluoroalkyl amide (manufactured by SeimiChemical Co., Ltd., Surflon (registered trademark) S-145), and 300 partsof water was stirred to give a protective layer coating composition.

Preparation of Heat-Sensitive Recording Material

The backside layer coating composition was applied, in an amount of 4g/m² on dry weight basis, to one side (backside) of a blue transparentpolyethylene terephthalate film (trade name: Melinex (registeredtrademark) 914, manufactured by Teijin DuPont Films Japan Limited,thickness: 175 μm, haze value: 3%) and dried to form a backside layer.The heat-sensitive recording layer coating composition and theprotective layer coating composition were successively applied to theother side (front side) of the film in amounts of 23 g/m² and 4 g/m²,respectively, on dry weight basis, and dried to form a heat-sensitiverecording layer and a protective layer, thereby giving a heat-sensitiverecording material.

EXAMPLE 2

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, spherical resin particles having a mean volumeparticle diameter of 8 μm (manufactured by Ganz Chemical Co., Ltd., GanzPearl (registered trademark) GM-0801, polymethylmethacrylate) were usedin an amount of 3.5 parts instead of 0.5 parts.

EXAMPLE 3

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, spherical resin particles having a mean volumeparticle diameter of 8 μm (manufactured by Ganz Chemical Co., Ltd., GanzPearl (registered trademark) GM-0801, polymethylmethacrylate) were usedin an amount of 0.3 parts instead of 0.5 parts.

EXAMPLE 4

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, spherical resin particles having a mean volumeparticle diameter of 8 μm (manufactured by Ganz Chemical Co., Ltd., GanzPearl (registered trademark) GM-0801, polymethylmethacrylate) were usedin an amount of 5.0 parts instead of 0.5 parts.

EXAMPLE 5

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording material, the backside layer coating composition was appliedin an amount such that the coating composition after being dried had anamount of 0.6 g/m² instead of 4 g/m².

EXAMPLE 6

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording material, the backside layer coating composition was appliedin an amount such that the coating composition after being dried had anamount of 8 g/m² instead of 4 g/m².

EXAMPLE 7

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording material, the backside layer coating composition was appliedin an amount such that the coating composition after being dried had anamount of 0.3 g/m² instead of 4 g/m².

EXAMPLE 8

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording material, the backside layer coating composition was appliedin an amount such that the coating composition after being dried had anamount of 12 g/m² instead of 4 g/m².

EXAMPLE 9

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 185 parts of an acrylic acid-acrylic acid estercopolymer latex having a glass transition temperature of 33° C.(manufactured by Saiden Chemical Industry, Saibinol (registeredtrademark) X-500-280E, solids content 46%) and 240 parts of water wereused in place of 425 parts of the core-shell latex (manufactured byMitsui Chemicals, Inc., Barriastar (registered trademark) B-1000, solidscontent: 20%).

EXAMPLE 10

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 275 parts of an acrylic acid-acrylic acid estercopolymer latex having a glass transition temperature of 88° C.(manufactured by Saiden Chemical Industry, Saibinol (registeredtrademark) EK-106, solids content 31%) and 150 parts of water were usedin place of 425 parts of the core-shell latex (manufactured by MitsuiChemicals, Inc., Barriastar (registered trademark) B-1000, solidscontent: 20%).

EXAMPLE 11

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 220 parts of an acrylic acid-acrylic acid estercopolymer latex having a glass transition temperature of 10° C.(manufactured by Saiden Chemical Industry, Saibinol (registeredtrademark) EK-32, solids content 39%) and 205 parts of water were usedin place of 425 mass parts of the core-shell latex (manufactured byMitsui Chemicals, Inc., Bariastar (registered trademark) B-1000, solidscontent: 20%).

EXAMPLE 12

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 75 parts of a core-shell latex (manufactured byMitsui Chemicals, Inc., Bariastar (registered trademark) B-1000, solidscontent: 20%) was used in place of 75 parts of the urethane-based resinlatex (manufactured by Dainippon Ink & Chemicals, Inc., Hydran(registered trademark) AP-30F, solids content: 20%).

EXAMPLE 13

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 0.5 parts of spherical resin particles having amean volume particle diameter of 4 μm (manufactured by Ganz ChemicalCo., Ltd., Ganz Pearl (registered trademark), polymethylmethacrylate)was used in place of 0.5 parts of the spherical resin particles having amean volume particle diameter of 8 μm (manufactured by Ganz ChemicalCo., Ltd., Ganz Pearl (registered trademark) GM-0801,polymethylmethacrylate).

EXAMPLE 14

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 0.5 parts of spherical resin particles having amean volume particle diameter of 10 μm (manufactured by Ganz ChemicalCo., Ltd., Ganz Pearl (registered trademark), polymethylmethacrylate)was used in place of 0.5 parts of the spherical resin particles having amean volume particle diameter of 8 μm (manufactured by Ganz ChemicalCo., Ltd., Ganz Pearl (registered trademark) GM-0801,polymethylmethacrylate).

EXAMPLE 15

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the protective layercoating composition, 250 parts of an 8% aqueous solution ofacetoacetyl-modified polyvinyl alcohol (manufactured by Nippon SyntheticChemical Industry Co., Ltd., Gohsefimer (registered trademark) OKS-3431,degree of polymerization: about 2300, degree of saponification: about 98mol %) was used in place of 100 parts of the ionomeric urethane-basedresin latex (manufactured by Dainippon Ink & Chemicals Inc., Hydran(registered trademark) AP-30F, solids content: 20%).

EXAMPLE 16

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the protective layercoating composition, 500 parts of an 8% aqueous solution ofacetoacetyl-modified polyvinyl alcohol (manufactured by Nippon SyntheticChemical Industry Co., Ltd., Gohsefimer (registered trademark) Z-200,degree of polymerization: about 1000, degree of saponification: about 98mol %) was used in place of 500 parts of the 8% aqueous solution ofacetoacetyl nodified polyvinyl alcohol (manufactured by Nippon SyntheticChemical Industry Co., Ltd., Gohsefimer (registered trademark) OKS-3431,degree of polymerization: about 2300, degree of saponification: about 98mol %).

EXAMPLE 17

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the protective layercoating composition, 79 parts of a 10% aqueous solution of anethyleneoxide adduct of perfluoroalkyl amide (manufactured by SeimiChemical Co., Ltd., Surflon (registered trademark) S-145) was used inplace of 26 parts of stearamide (manufactured by Chukyo Yushi Co., Ltd.,Hymicron L271, solids content: 25%) and 4 parts of potassium stearylphosphate (manufactured by Matsumoto Yushi Seiyaku, Woopol (registeredtrademark) 1800, solids content: 35%).

EXAMPLE 18

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the protective layercoating composition, 16 parts of a polyethylene wax (manufactured by SanNopco Limited, Nopcote (registered trademark) PEM-17, solids content:40%) was used in place of 26 parts of stearamide (manufactured by ChukyoYushi Co., Ltd., Hymicron L271, solids content: 25%).

EXAMPLE 19

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording layer coating composition, 20 parts of astyrene-butadiene-based latex (manufactured by Nippon A&L Inc., solidscontent: 48%, Smartex (registered trademark) PA9281) was used in placeof 50 parts of ionomeric urethane-based resin latex (manufactured byDainippon Ink & Chemicals, Inc., Hydran (registered trademark) AP-30F,solids content: 20%).

EXAMPLE 20

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording layer coating composition, 75 parts of an ionomericurethane-based resin latex (manufactured by Dainippon Ink & Chemicals,Inc., Hydran (registered trademark) AP-30F, solids content: 20%) wasused in place of 30 parts of the styrene-butadiene-based latex(manufactured by Nippon A&L Inc., solids content: 48%, Smartex(registered trademark) PA9281).

EXAMPLE 21

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the heat-sensitiverecording layer coating composition, 61 parts of a latex having a solidscontent of 41% and prepared by polymerizing styrene monomer andbutadiene monomer in an aqueous medium containing a polyurethane ionomer(Patelacol (registered trademark) 2090, manufactured by Dainippon Ink &Chemicals, Inc.) was used in place of 30 parts of thestyrene-butadiene-based latex (manufactured by Nippon A&L Inc., solidscontent: 48%, Smartex (registered trademark) PA9281) and 50 parts of theionomeric urethane-based resin latex (manufactured by Dainippon Ink &Chemicals, Inc., Hydran (registered trademark) AP-30F, solids content:20%).

COMPARATIVE EXAMPLE 1

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, spherical resin particles having a mean volumeparticle diameter of 8 μm (manufactured by Ganz Chemical Co., Ltd., GanzPearl (registered trademark) GM-0801, polymethylmethacrylate) were usedin an amount of 0.1 parts instead of 0.5 parts.

COMPARATIVE EXAMPLE 2

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, spherical resin particles having a mean volumeparticle diameter of 8 μm (manufactured by Ganz Chemical Co., Ltd., GanzPearl (registered trademark) GM-0801, polymethylmethacrylate) were usedin an amount of 8.0 parts instead of 0.5 parts.

COMPARATIVE EXAMPLE 3

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 0.5 parts of spherical resin particles having amean volume particle diameter of 20 μm (manufactured by Ganz ChemicalCo., Ltd., Ganz Pearl (registered trademark), polymethylmethacrylate)was used in place of 0.5 parts of the spherical resin particles having amean volume particle diameter of 8 μm (manufactured by Ganz ChemicalCo., Ltd., Ganz Pearl (registered trademark) GM-0801,polymethylmethacrylate).

COMPARATIVE EXAMPLE 4

A heat-sensitive recording material was produced in the same manner asin Example 1 except that, in the preparation of the backside layercoating composition, 0.5 parts of spherical resin particles having amean volume particle diameter of 1 μm was used in place of 0.5 parts ofspherical resin particles having a mean volume particle diameter of 8 μm(manufactured by Ganz Chemical Co., Ltd., Ganz Pearl (registeredtrademark) GM-0801, polymethylmethacrylate).

[Evaluation of Heat-Sensitive Recording Materials]

The heat-sensitive recording materials obtained above were evaluated asfollows. Table 1 shows the results.

Blocking Resistance

Five pieces (10 cm×10 cm each) of a heat-sensitive recording materialwere placed one on top of another such that the front side of each piecefaced upward. A 200 g copper plate (10 cm×10 cm) was placed on theuppermost piece, and the heat-sensitive recording materials were left tostand at 40° C. at 90% RH for 7 days. The extent of the resistance toblocking between the backside of the second piece from the top and thefront side of the third piece from the top was visually examined.

-   -   ⊚: No trace of blocking observable on the front side of the        heat-sensitive recording material    -   ◯: Trace of blocking minimally observable on the front side of        the heat-sensitive recording material    -   Δ: Traces of blocking somewhat observable on the front side of        the heat-sensitive recording material    -   ×: Many traces of blocking observable on the front side of the        heat-sensitive recording material        Multi-Feeding Resistance

Forty A4-sized pieces of heat-sensitive recording material were left tostand at 23° C. and 50% RH for 2 hours. These pieces were then subjectedto recording by means of a thermal printer (trade name: NP1660M,manufactured by CODONICS) while examining the extent of themulti-feeding resistance thereof.

-   -   ⋆: No multi-feeding    -   ⊚: Multi-feeding once    -   ◯: Multi-feeding two or three times    -   ×: Multi-feeding at least four times        Curl Resistance

An A4-sized piece of heat-sensitive recording material was horizontallyplaced with the inwardly curled surface facing upward. The height of its4 corners was measured, and the average value thereof (mm) was referredto as curl value; the smaller the curl value, the better the resistanceto curling. Note that curling created in the direction of the recordingsurface is indicated by “+”, and curling created in the direction of thebackside is indicated by “−”. The measurement of curl values prior torecording was carried out with respect to the heat-sensitive recordingmaterials that had been left to stand at 23° C. and 15% RH for 2 hours,and also with respect to the heat-sensitive recording material that hadbeen left to stand at 23° C. and 50% RH for 2 hours. The curl valueafter recording was measured as follows: heat-sensitive recordingmaterials that had been left to stand at 23° C. and 50% RH for 2 hourswere subjected to recording by means of a thermal printer (trade name:NP1660M, manufactured by Codonics, Inc.), and immediately thereafterleft to stand at 23° C. and 15% RH for 30 minutes or at 23° C. and 50%RH for 30 minutes, and the curl value of each heat-sensitive recordingmaterial was measured.

Coefficient of Friction

The coefficient of static friction between the front side and thebackside of a heat-sensitive recording material was measured accordingto ASTM D4521-96 (horizontal plane method)

Thickness of Backside Layer

The thickness (μm) of the backside layer of a heat-sensitive recordingmaterial was calculated from an electron micrograph of the cross sectionof the recording material.

Haze Value

The haze value of a heat-sensitive recording material was measuredaccording to JIS K 7136 by a haze meter (TC-H IV, manufactured by TokyoDenshoku).

Resistance to Residual Substance Accumulation on Thermal Head

Recording was conducted using a thermal printer (UP-930, manufactured bySony Corporation) over a length of 5 m. Residual substance accumulationon the thermal head was visually examined.

-   -   ⊚: No observable residual substance accumulation on the thermal        head    -   ◯: Residual substance accumulation on the thermal head somewhat        observable    -   ×: Considerable residual substance accumulation on the thermal        head observable        Resistance to Surface Roughing and Glossiness of Recorded        Portions

The glossiness of unrecorded portions and recorded portions produced bya thermal head at an energy of 30 mJ/mm² (low energy) or 80 mJ/mm² (highenergy) (resistance: 520 Ω, 8 dots/mm, 0.015 mm²/dot, applied pulsewidth: 2 m sec, applied pulse cycle: 5 m sec, line pressure: 0.02MPa/cm) was measured using a gloss meter (product name: GM-26D,manufactured by Murakami Color Research Laboratory) with an incidenceangle of 75°.

Recorded portions produced at high energy were visually inspected forresistance to surface roughing and evaluated as follows.

-   -   ⊚: Surface of recorded portion was scarcely roughened.    -   ◯: Surface of recorded portion was slightly roughened.    -   ×: Surface of recorded portion was severely roughened.        Blurring Resistance

The above-mentioned recorded portion produced at high energy wasvisually checked for blurring and evaluated as follows.

-   -   ⊚: No blurring at the edges of recorded portions    -   ×: Blurring at the edges of recorded portions

TABLE 1 Backside layer Curl Resistance (mm) Resin Before particles ResinTg of recording After recording mean particles Thick- main (23° C.) (23°C.) Coefficient Blocking diameter content ness binder 50% 15% 50% 15% ofstatic resist- (μm) (%) (μm) (° C.) RH RH RH RH friction ance Ex. 1 80.5 4 218 +1 +4 +4 −4 0.20 ⊚ Ex. 2 8 3.4 4 218 +1 +4 +4 −4 0.18 ⊚ Ex. 38 0.3 4 218 +1 +4 +4 −4 0.22 ⊚ Ex. 4 8 4.8 4 218 +1 +4 +4 −4 0.18 ⊚ Ex.5 8 0.5 0.6 218 +2 +5 +5 0 0.20 ⊚ Ex. 6 8 0.5 8 218 −1 +1 +2 −3 0.20 ⊚Ex. 7 8 0.5 0.3 218 +4 +6 +7 +1 0.25 ⊚ Ex. 8 8 0.5 12 218 −2 +2 +1 −50.23 ◯ Ex. 9 8 0.5 4 33 +2 +5 +6 +2 0.25 ⊚ Ex. 10 8 0.5 4 88 +1 +5 +5 −30.22 ⊚ Ex. 11 8 0.5 4 10 0 +5 +7 +5 0.28 ◯ Ex. 12 8 0.5 4 218 +1 +4 +4−3 0.20 ◯ Ex. 13 4 0.5 4 218 +1 +4 +4 −4 0.23 ⊚ Ex. 14 10 0.5 4 218 +1+4 +4 −4 0.18 ⊚ Ex. 15 8 0.5 4 218 +1 +4 +4 −4 0.20 ⊚ Ex. 16 8 0.5 4 218+1 +4 +4 −4 0.20 ⊚ Ex. 17 8 0.5 4 218 +1 +4 +4 −4 0.20 ⊚ Ex. 18 8 0.5 4218 +1 +4 +4 −4 0.20 ⊚ Ex. 19 8 0.5 4 218 +1 +4 +4 −4 0.20 ⊚ Ex. 20 80.5 4 218 +1 +4 +4 −4 0.20 ⊚ Ex. 21 8 0.5 4 218 +1 +4 +4 −4 0.20 ⊚ Comp.Ex. 1 8 0.1 4 218 +1 +4 +4 −4 0.31 ◯ Comp. Ex. 2 8 7.4 4 218 +1 +4 +4 −40.26 ⊚ Comp. Ex. 3 20 0.5 4 218 +1 +4 +4 −4 0.25 Δ Comp. Ex. 4 1 0.5 4218 +1 +4 +4 −4 0.32 X Resistance to Resistance Multi- adhesion of tosurface feeding residual roughing of Glossiness (%) Blurring resist-Haze substance to recorded Unrecorded At low At high resist- ance valuehead portion portion energy energy ance Ex. 1 ⋆ 34% ⊚ ⊚ 91 92 94 ◯ Ex. 2⋆ 38% ⊚ ⊚ 91 92 94 ◯ Ex. 3 ⋆ 34% ⊚ ⊚ 91 92 94 ◯ Ex. 4 ⋆ 40% ⊚ ⊚ 91 92 94◯ Ex. 5 ⋆ 32% ⊚ ⊚ 91 92 94 ◯ Ex. 6 ⋆ 38% ⊚ ⊚ 91 92 94 ◯ Ex. 7 ◯ 32% ⊚ ⊚91 92 94 ◯ Ex. 8 ⊚ 41% ⊚ ⊚ 91 92 94 ◯ Ex. 9 ⋆ 33% ⊚ ⊚ 91 92 94 ◯ Ex. 10⋆ 34% ⊚ ⊚ 91 92 94 ◯ Ex. 11 ◯ 33% ⊚ ⊚ 91 92 94 ◯ Ex. 12 ⋆ 34% ⊚ ⊚ 91 9294 ◯ Ex. 13 ⋆ 35% ⊚ ⊚ 91 92 94 ◯ Ex. 14 ⋆ 34% ⊚ ⊚ 91 92 94 ◯ Ex. 15 ⋆32% ⊚ ⊚ 93 93 91 ◯ Ex. 16 ⋆ 34% ⊚ X 90 91 80 ◯ Ex. 17 ⋆ 31% X ◯ 94 94 93◯ Ex. 18 ⋆ 33% ◯ ⊚ 92 91 93 ◯ Ex. 19 ⋆ 32% ⊚ ⊚ 91 92 94 X Ex. 20 ⋆ 33% ⊚⊚ 91 92 94 X Ex. 21 ⋆ 32% ⊚ ⊚ 91 92 94 ◯ Comp. Ex. 1 X 31% ⊚ ⊚ 91 92 94◯ Comp. Ex. 2 Δ 52% ⊚ ⊚ 91 92 94 ◯ Comp. Ex. 3 Δ 32% ⊚ ⊚ 91 92 94 ◯Comp. Ex. 4 X 31% ⊚ ⊚ 91 92 94 ◯

INDUSTRIAL APPLICABILITY

As shown in Table 1, the heat-sensitive recording material of theinvention has the effect of preventing the blocking that is caused byadhesion of the front side and backside of the heat-sensitive recordingmaterial even when exposed to conditions of 40° C. and 90% RH.

1. A heat-sensitive recording material comprising: (a) a transparentfilm; (b) a heat-sensitive recording layer formed on one side of thetransparent film, and containing an electron-donating compound, anelectron-accepting compound, and a binder, the binder being aurethane-based resin and a styrene-butadiene-based resin; (c) aprotective layer formed on the heat-sensitive recording layer, andcontaining an aqueous resin as a main ingredient, the aqueous resinbeing an acetoacetyl-modified polyvinyl alcohol having a polymerizationdegree of 1500 to 3000 and a saponification degree of at least 95 mol %,the protective layer further containing a fluorine-containing surfactantand at least one compound selected from the group consisting of alkylphosphate salts, waxes and higher fatty acid amides; and (d) a backsidelayer formed on the other side of the transparent film, and containing apigment and a binder, the heat-sensitive recording material containing,as the pigment, in the backside layer spherical resin particles having amean volume particle diameter of 2 to 15 μm in an amount of 0.2 to 5.0mass % of the backside layer, the average thickness of the backsidelayer being 0.5 to 10 μm and being less than or equal to the mean volumeparticle diameter of the spherical resin particles contained in thebackside layer, and the binder in the backside layer being a(meth)acrylamide-based resin binder having a glass transitiontemperature of 180 to 250° C. and an ionomeric urethane-based resin, theionomeric urethane-based resin being contained in an amount of 3 to 30mass % of all the binders.
 2. A heat-sensitive recording materialaccording to claim 1, wherein the average thickness of the backsidelayer is 0.5 to 10 μm and is less than the mean volume particle diameterof the spherical resin particles contained in the backside layer.
 3. Aheat-sensitive recording material according to claim 1, wherein the(meth)acrylamide-based resin binder in the backside layer is acore-shell-structured latex.
 4. A heat-sensitive recording materialaccording to claim 1, wherein the (meth)acrylamide-based resin binderhas a glass transition temperature of 200 to 230° C.
 5. A heat-sensitiverecording material according to claim 1, wherein the ionomericurethane-based resin is contained in an amount of 5 to 20 mass % of allthe binders.
 6. A heat-sensitive recording material according to claim1, wherein the protective layer further contains an ionomericurethane-based resin as an aqueous resin.
 7. A heat-sensitive recordingmaterial according to claim 6, wherein the ionomeric urethane-basedresin is present in an amount of 10 to 60 mass % relative to theacetoacetyl-modified polyvinyl alcohol.
 8. A heat-sensitive recordingmaterial according to claim 1, wherein the total amount of thefluorine-containing surfactant and said at least one compound selectedfrom the group consisting of alkyl phosphate salts, waxes and higherfatty acid amides in the protective layer is 0.5 to 15 mass % of theprotective layer.
 9. A heat-sensitive recording material according toclaim 1, wherein said at least one compound selected from the groupconsisting of alkyl phosphate salts, waxes and higher fatty acid amidesis present in an amount of 50 to 800 mass % relative to thefluorine-containing surfactant in the protective layer.
 10. Aheat-sensitive recording material according to claim 1, wherein theprotective layer contains an alkyl phosphate salt, a fluorine-containingsurfactant, and a compound selected from the group consisting of waxesand higher fatty acid amides.
 11. A heat-sensitive recording materialaccording to claim 1, wherein the protective layer contains an alkylphosphate salt, a higher fatty acid amide and a fluorine-containingsurfactant.
 12. A heat-sensitive recording material according to claim1, wherein the binder in the heat-sensitive recording layer is anionomeric urethane-based resin and a styrene-butadiene-based resin. 13.A heat-sensitive recording material according to claim 1, wherein thestyrene-butadiene-based resin is present in an amount of 100 to 300 massparts per 100 mass parts of the ionomeric urethane-based resin.
 14. Aheat-sensitive recording material according to claim 1, wherein theelectron-donating compound in the heat-sensitive recording layer is aleuco dye, and the leuco dye is microencapsulated in a resin film or isin the form of a resin composite particle containing the leuco dye. 15.A heat-sensitive recording material according to claim 1, wherein theheat-sensitive recording layer has a thickness of 15 to 30 μm.
 16. Aheat-sensitive recording material according to claim 1, wherein thetransparent film is a polyethylene terephthalate film having a thicknessof 40 to 250 μm.
 17. A heat-sensitive recording material according toclaim 1 having a haze value of 10 to 50%.